Quantum mechanics was developed as an abstract theory of particles and fields, but it is now understood that it also constitutes the foundation for a novel type of technology, where quantum objects are used as carriers of information. Such quantum technology has revolutionary application prospects for quantum simulations impossible with classical computers, unbreakable quantum cryptography, and quantum sensors with unprecedented capabilities. Optical quantum technology is currently undergoing a revolution due to progress in semiconductor nanotechnology, which on one hand allows studying hitherto unseen synthetic quantum systems and on the other render the production of commercial optical quantum technology viable.
Data on everyday computers are comprised of bits, which are a binary sequence of zeroes and ones. These bits can be stored magnetically on hard drives or electrically on flash drives. In the last few decades quantum information processing using quantum bits or “qubits” has emerged as a completely new form of computation and as previously mentioned carriers of information. Unlike a bit, a single qubit is a quantum mechanical object and can be in a combination or superposition of zero and one states. Qubits can be manipulated and processed to perform computational tasks.
Two common ways to represent a qubit include: using the quantized angular momentum, or spin, of a charged particle, e.g. spin up=0 and spin down=1, or by using photons, e.g. one photon in one particular optical mode=0, while one photon present in a separate mode=1.
Thus, the fundamental resource for photonic quantum technology is a single particle of light, i.e. a photon. However, generating and controlling single photons are challenging tasks. Photonic nanostructures, such as photonic crystals, are particularly useful for this purpose and remarkable progress has been made over the past decade. A key goal is to generate single photons on demand, where the photons are coupled with near-unity efficiency into a single mode and subsequently couple the photons with high efficiency into an optical fibre. Such a highly efficient and deterministic single-photon source could be used for photonic quantum simulators or to establish a new metrological measure of the standard of light intensity, the candela. It could also significantly increase the performance of quantum cryptography systems, where current technology is based on attenuated lasers, which suffer from an intrinsically small average photon number.
It has been suggested that semiconductor quantum dots (QDs) are excellent candidates for stationary qubits. However, many of the prior art systems using quantum light sources, such as QDs, suffer from a low coupling efficiency of about 10% or even lower. However, in order to commercialise photonic devices capable of generating or otherwise processing single photons, it is crucial that the coupling efficiency of the single photons from a single photon emitter and on to an optical fibre is as high as possible and preferably near unity. This enables generating a triggered source of single photons that can be multiplexed. Therefore, the figure of merit for quantum simulation is the efficiency to the Nth power, where N is the number of photons in separate optical modes. About N=40 photons are needed to carry out quantum simulations that cannot be done on classical computers and the scaling of the complexity of problems that can be addressed is dramatic since the built-in parallelism in quantum mechanics is exploited.
For example, 40 photon channels (qubits) correspond to 4 TB of classical information. Taking 40 channels as a benchmark and assuming a 10% efficiency, the probability of creating a useful photonic state is thus 10^(−40), which is a very low probability. With a 90% efficiency, the probability is 1%, and with a 95% efficiency, the success probability even for 80 channels is around 2%. With a MHz repetition rate of generating the single photons, 1-2% efficiency is easily enough to do calculations. To give an idea about the potential of an 80-qubit quantum simulator, 80 qubit quantum channels have a storage capacity of 10,000 times the amount of information stored by mankind.
Peter Lodahl et al.: “Interfacing single photons and single quantum dots with photonic nanostructures”, 4 Dec. 2013, ArXiv, is a disclosure by the present inventor group that provides an overview of quantum optics with excitons in single quantum dots embedded in photonic nanostructures.
Accordingly, it is seen that there is a need for optical devices which with a high efficiency can couple single-photons from a single-photon source to an optical fibre or alternatively couple single-photons from an optical fibre to a single-photon detector.